In-plane microwave dielectric properties of paraelectric barium strontium titanate thin films with anisotropic epitaxy

In-plane dielectric properties of 〈110〉 oriented epitaxial (Ba0.60 Sr0.40) Ti O3 thin films in the thickness range from 25-1200 nm have been investigated under the influence of anisotropic epitaxial strains from 〈100〉 NdGa O3 substrates. The measured dielectric properties show strong residual strain and in-plane directional dependence. Below 150 nm film thickness, there appears to be a phase transition due to the anisotropic nature of the misfit strain relaxation. In-plane relative permittivity is found to vary from as much as 500-150 along [1 1- 0] and [001] respectively, in 600 nm thick films, and from 75 to 500 overall. Tunability was found to vary from as much as 54% to 20% in all films and directions, and in a given film the best tunability is observed along the compressed axis in a mixed strain state, 54% along [1 1- 0] in the 600 nm film for example.